Opaque types v.s. Boxed types

opaque_type_vs_boxed_type.swift

protocol Shape {
    associatedtype T
    func draw() -> String
}

struct Triangle: Shape {
    typealias T = Int
    
    var size: Int
    func draw() -> String {
       var result: [String] = []
       for length in 1...size {
           result.append(String(repeating: "*", count: length))
       }
       return result.joined(separator: "\n")
    }
}

struct Square: Shape {
    typealias T = Int
    
    var size: Int
    func draw() -> String {
        let line = String(repeating: "*", count: size)
        let result = Array<String>(repeating: line, count: size)
        return result.joined(separator: "\n")
    }
}

// Opaque types
// Compile error: Conflicting arguments to generic parameter 'τ_0_0' ('Square' vs. 'Triangle')
var tmp1: [some Shape] = [Square(size: 1), Triangle(size: 1)] //[Square(size: 1), Square(size: 2)]

// Boxed types
var tmp2: [any Shape] = [Square(size: 1), Triangle(size: 1)]